1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases from automobiles, and more particularly, to a catalyst which can efficiently purify nitrogen oxides (NO.sub.x) in exhaust gases in oxygen-rich atmospheres whose oxygen concentrations are more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of the Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x simultaneously into innocuous entities. For example, 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal catalyst selected from the group consisting of Pt, Pd, and Rh and loaded on the catalyst carrier layer.
The purifying performance of these 3-way catalysts for purifying exhaust gases largely depends on the air-fuel ratio (A/F) of automotive engines. For instance, when the air-fuel ratio is large, i.e., when the fuel concentration is small (or on the fuel-lean side), the oxygen concentration is large in exhaust gases. Accordingly, the reactions of oxidizing CO and HC are active, but the reactions of reducing NO.sub.x are inactive. On the other hand, when the air-fuel ratio is small, i.e., when the fuel concentration is high (or on the fuel-rich side), the oxygen concentration is small in exhaust gases. Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Accordingly, the air-fuel ratio varies frequently in a range from the values adjacent to the stoichiometric point (or the ideal air-fuel ratio) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving in urban areas, it is necessary to operate automobiles on the fuel-lean side where the air-fuel mixture containing as excessive oxygen as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
In view of the aforementioned circumstances, the applicants of the present invention applied for a Japanese Patent for a novel catalyst under Japanese Unexamined Patent Publication (KOKAI) No.5-317,652. This catalyst comprises a support, and an alkaline-earth metal and Pt loaded on the support. With this support, NO.sub.x are adsorbed on the alkaline-earth metal when the air-fuel ratios are on the fuel-lean side (i.e., in the oxygen-rich atmospheres) driving. Then, when the automobiles are driven under the air-fuel ratios at the stoichiometric point or on the fuel-rich side (i.e., in the oxygen-lean atmospheres), NO.sub.x are purified by reacting with reducing gases such as HC and the like. Thus, this catalyst is superb in the NO.sub.x purifying performance even on the fuel-lean side (i.e., in the oxygen-rich atmospheres).
This catalyst is believed to adsorb NO.sub.x by the following mechanism: The alkaline-earth metal, for example, Ba is loaded on the support in a form of the simple oxide. The simple oxide of Ba reacts with NO.sub.x to produce barium nitrate (Ba(NO.sub.3).sub.2). Thus, NO.sub.x are adsorbed on the support of the catalyst.
However, the exhaust gases usually contain SO.sub.2 which is produced by burning sulfur (S) contained in the fuel. The catalyst ingredient further oxidizes SO.sub.2 to SO.sub.3 in the oxygen-rich atmospheres (i.e., on the fuel-lean side). Then it has become apparent that SO.sub.3 reacts readily with water also contained in the exhaust gases to produce sulfuric acid, and that the resulting sulfuric acid reacts with Ba to produce sulfite and/or sulfate, which do not adsorb NO.sub.x. When Ba or the like is converted into sulfite or sulfate, it cannot adsorb NO.sub.x any more. As a result, this catalyst suffers from a drawback in that it is deteriorated in terms of the NO.sub.x purifying performance after it has been used for a long time.